The present invention relates to a new and improved pump assembly having a positive displacement pump and a centrifugal pump.
Pump assemblies which include positive displacement and centrifugal pumps are used to supply fuel to aircraft engines. During starting and low speed operation of the engine, the positive displacement pump is effective to supply fuel to the engine. As the engine speed increases, the centrifugal pump supplies the increased demand for fuel by the engine. Various pump assemblies having centrifugal and positive displacement pumps for supplying fuel to an engine are disclosed in U.S. Pat. Nos. 3,851,998; 3,941,505; and 4,247,263.
The pump assembly disclosed in U.S. Pat. No. 3,851,998 includes a positive displacement pump having vane elements which are mounted on an impeller of a centrifugal pump. During low speed operation of the pump assembly, the vane elements are biased inwardly agaist a stationary cam surface by springs and fluid pressure. When a predetermined idle speed is reached, a change-over valve is operated to throttle the flow of inlet fluid to the positive displacement pump and direct the fluid to the main or centrifugal pump. As the flow of fuel to the positive displacement pump is blocked, the fluid pressure urging the vanes into engagement with the stationary cam surface is vented so that the vanes are free to move outwardly under the influence of centrifugal force. This reduces the heat generated by the positive displacement pump and the power required to operate the pump assembly.